High frequency components are used in various electronic applications operating in high frequency ranges, especially in the radio frequency range (RF) (3 kHz–300 MHz) or microwave frequency range (300 MHz–30 GHz). In practice the term high frequency is typically used for frequencies higher than 3 MHz. Besides discrete components high frequency components may be parts of integrated electronic circuits. Electronic circuits typically comprise integrated circuits and active and passive components connected to the circuits. Integrated RF and microwave circuits can be divided into hybrid circuits and monolithic microwave circuits (MMIC), which can both be used to manufacture layered structures comprising conductors and components in several layers. High frequency components themselves can also comprise other components such as resistors, amplifiers or capacitors, typically implemented as discrete components.
Nowadays multilayered printed boards with ten or more layers can be manufactured. An advantage of layered structures is the increased routing space and new possibilities of designing or minimizing the devices. In multilayered printed boards the possibility of high coupling between the transmission lines is a feature often seen as a disadvantage. However, in some implementations, such as directional couplers, coupling can be exploited. A disadvantage with layered structures made of different dielectric materials with different dielectric constants, whereby the transmission lines are placed on a surface of the printed board, is the degraded directivity caused by differences in the propagation mode velocities due to non-homogeneity of the dielectric media.
A directional coupler is an example of a high frequency component used in RF and microwave applications for various purposes, such as power monitoring or sampling or power division. Directional couplers can be implemented using coupled lines, typically microstrips, which are coupled together to form a microstrip directional coupler. A problem with the microstrip directional couplers is that directivity is poor, and it decreases with frequency. Eventually the directivity can become negative, resulting to a situation where the signal in the port, which was meant to be isolated can become stronger than the signal in the coupled port of the directional coupler. Besides potential non-homogeneities of the structure, this is derived from the basic structure of a microstrip configuration with a single ground plane only on one side, and the unequal velocities of the propagation modes in microstrip coupled lines.
In prior art, discrete capacitors have been used to improve the directivity. However, the solution suffers from high tolerances caused by a large spread of capacitance values when several separate capacitors are used. The use of separate capacitor components also increases the manufacturing costs, makes the manufacturing more complicated and decreases the reliability of the component because of the needed soldered joints, for example.
In prior art, interdigital or gap capacitors have also been introduced to be used for improving the directivity of a directional coupler. The interdigital capacitor solution is based on capacitors formed by arranging two or four fingers of two coupled lines side by side and crosswise to the direction of the coupled lines. The gap capacitors are based on series of gaps in a microstrip conductor between two coupled lines. However, a problem with these solutions is the limited value of achievable capacitance, even when substrate materials with high dielectric permittivity, i.e. high values of dielectric constants, are used.
In prior art, there are theories for compensating the velocity of even and odd propagation modes in broadside coupled stripline structures, for very strong couplings where the even mode velocity is higher than the odd mode velocity, by incorporating capacitors at the edges of the transmission lines, which capacitors are located between the transmission lines and the ground. In the technique the capacitors are shorted to the ground by using a via connection. A problem with this technique is, however, that it makes the structure more complicated to manufacture, as the used capacitors are connected to the ground, which requires via holes to be made. The technique is well suited to broadside couplers, where the coupling takes place in relation to the broad side of the transmission lines, but not to edged couplers, where the transmission lines are coupled in relation to the narrow side of the transmission lines.